A basic technique of vector control on an induction motor using an inverter is a prior art that has been used extensively in the industrial field. This technique is to control a torque of a motor instantaneously at a high speed through an operation of a torque component current in an orthogonal relation to a secondary magnetic flux inside the motor by operating the magnitude and the phase of an inverter output voltage separately.
The vector control of an induction motor is a technique that is being used also in the electric railroad in recent years.
A driving inverter of an electric vehicle is characterized in that the switching mode of the inverter is switched in such a manner that a multi-pulse PWM mode, which is employed generally in many cases, is used in a low speed range and a single-pulse mode is used in a medium and high speed range in which the inverter output voltage saturates and is fixed to the maximum value.
The multi-pulse PWM (pulse width modulation) mode referred to herein is a generally well-known PWM method and it is a mode to generate a PWM signal by comparing a triangular wave at a frequency of about 1 kHz with a voltage command.
The single-pulse mode referred to herein is to shape an output line-to-line voltage of the inverter to the waveform of 120° rectangular-wave conduction. Because the effective value of the fundamental wave of an inverter output voltage can be increased to the maximum and the number of pulses in a half cycle of the output voltage fundamental wave can be reduced to one, which is the minimum, it is characterized in that a compact and light inverter can be obtained by minimizing a switching loss of the inverter and making a cooling device smaller.
The waveform of 120° rectangular-wave conduction referred to herein is a voltage waveform by which a line-to-line voltage of the inverter has one pulse in a half cycle and a conduction width is 120° in electric angle.
For the inverter in an electric vehicle, it is essential to have the capability of performing stable vector control over the entire range from the multi-pulse PWM mode in a low speed range to the single-pulse mode in a medium and high speed range in which an output voltage of the inverter saturates and is fixed to the maximum value, and a vector control technique in an output voltage saturation range of the inverter and a pulse mode switching technique are crucial elements.
In particular, the magnitude of an output voltage of the inverter is fixed to the maximum voltage corresponding to an input voltage of the inverter in the output voltage saturation range of the inverter. It is therefore necessary to devise a technique to establish vector control.
In the output voltage saturation range of the inverter, in a case where an inverter output voltage command computed by a vector control device exceeds the maximum voltage that the inverter can actually output, the inverter fails to output a voltage according to the inverter output voltage command.
Accordingly, there is a discrepancy between a secondary magnetic flux command to the induction motor and a secondary magnetic flux inside the motor, which makes it difficult to perform vector control appropriately.
In order to avoid such a phenomenon, it is necessary to adjust a secondary magnetic flux command so that an inverter output voltage command will not exceed the maximum voltage that the inverter can actually output.
To be more concrete, in a case where the inverter output voltage command exceeds the maximum voltage that the inverter can actually output, the inverter output voltage command has to be lowered by lowering the secondary magnetic flux command.
Non-Patent Document 1 specified below discloses a vector control method that solves the problems discussed above.
Non-Patent Document 1 discloses that the inverter output voltage command can be corrected so as to coincide with the maximum output voltage that the inverter can actually output and hence vector control is enabled even in the output voltage saturation range of the inverter by configuring in such a manner that when an inverter output voltage command computed by the vector control device exceeds the maximum voltage that the inverter can output, a difference between the inverter output voltage command and the voltage that the inverter can actually output is inputted to a magnetic flux correction controller, so that the secondary magnetic flux command is lowered by an output of the magnetic flux correction controller.
Non-Patent Document 1: “Denatsu kotei moudo deno yuuden dendouki no bekutoru seigyo”, Journal of IEEJ, Vol. 118-D, No. 9, 1998.